Typical hot runner valve gated nozzles in plastic molding systems use a single central melt channel to convey the resin from a supply channel in a hot runner manifold to the mold cavity gate. A valve stem is generally employed which is a small diameter rod that is positioned centrally in the nozzle's melt channel and reciprocates to control the opening and closing of the valve gate.
Hot runner valve gate systems use a variety of mechanisms to operate the valve stem. U.S. Pat. Nos. 4,786,246 and 3,241,191 use an external moving mold plate to operate the valve stem. U.S. Pat. No. 4,330,258 uses an externally operated rack to drive a gear which in turn via a thread moves the stem. U.S. Pat. No. 4,173,448 to Rees et al. uses a double acting air operated piston to move the stem. These references all positively move the stem to an open or closed position by using some external means operating on the stem.
Some devices, typically using springs, provide single acting forces to close the stem. Opening the stem is performed by the pressure of the resin in the nozzle assembly. Examples of this type are shown in U.S. Pat. Nos. 3,491,408 and 4,095,931, which uses a cam to close the stem. Further examples of this type of device miniaturize the stem/spring assembly to fit inside the nozzle. These are shown in U.S. Pat. Nos. 3,677,682 and 4,171,941.
All of the above devices are limited in size of gate opening that can be provided. Typically the valve stem must be operated to close the gate immediately after injection or hold pressures have been reached. These pressure values can be as much as 20,000 psi and accordingly the projected area of the valve stem, and hence the gate diameter it can close, is limited by the amount of external force that can be applied to close the stem against this resisting melt pressure. For example a typical 0.060" diameter gate at 20,000 psi will exert a force of 245 lbs. against a valve stem of 0.125" diameter. Thus an air piston operating at a nominal 80 psi air supply pressure must be approximately 2" diameter to develop sufficient force to close the valve. Obviously a larger diameter gate and stem would require greater forces, this in turn would require larger air piston diameters or using hydraulic pistons operating at higher fluid pressures, or alternately mechanical cam or rack and gear mechanisms. These alternate methods all have disadvantages. Hydraulic circuits in close proximity to heated mold components invite the risk of fire. Mechanical methods, like larger diameter air pistons, rack and gear drives and cams, take up a lot of space in the structure of the mold and are prone to seizing or failure since they must operate on components that thermally expand and physically move out of their cold condition alignment.
Large diameter valves, providing larger gate opening sizes, are shown in U.S. Pat. Nos. 4,808,106 and 5,002,480. The '106 reference relies on the deflection of the valve material itself to open and close the valve, whereas the '480 reference uses a sliding stem. The operation of the valve is performed by controlling the melt pressure itself. This greatly limits the usefulness of the valve since accurate melt pressure control in the midst of a molding cycle may not be accurate enough to effect opening or closing the valve at the optimum points in the cycle.
U.S. Pat. No. 5,098,280 shows a sliding gate needle contained within the hot runner nozzle. The needle is not a valve gate and cannot operate to shut off the resin flow, rather it is a heat conducting means to maintain the melt temperature in the gate area. The needle is able to slide back and forth within the nozzle by virtue of the melt pressure acting on each side of the stem's structure.
Accordingly, it is a principal object of the present invention to provide an improved injection molding apparatus which utilizes a pressure balanced valve stem.
It is a still further object of the present invention to provide an improved apparatus as aforesaid which is versatile, enables the use of large valve gates, does not result in delay of the molding cycle and enables precise control of the gate in a simple, convenient and expeditious manner.
Further objects and advantages of the present invention will appear hereinbelow.